Interaction of M3+ lanthanide cations with phosphoryl containing (alkyl)3PO versus (alkyl-O)3PO ligands: steric effects are more important than basicity effects †

Abstract

The concept of acid-base interactions between metallic cations and putative ligands, widely used in cation coordination and complexation chemistry, is discussed from the analysis of M 3ϩ lanthanide cation binding to phosphoryl ligands in the gas phase. We report quantum mechanical calculations on the effect of alkyl vs. O-alkyl substitution in phosphoryl-containing ligands L interacting with M 3ϩ (La 3ϩ , Eu 3ϩ , Yb 3ϩ ; L = Me 3 PO, Me 2 (MeO)PO, Me(MeO) 2 PO and (MeO) 3 PO). In the charged 1 : 1 LM 3ϩ complexes, O-alkyl substituted ligands yield lower interaction energies than the alkyl substituted ones, in agreement with the calculated gas phase basicities. However, the effect is surprisingly small (about 10 times smaller than the alkyl vs. aryl substituent effect). The comparison of LM 3ϩ with neutral LMCl 3 and L 2 MCl 3 complexes reveals an amplification of O-alkyl vs. alkyl weakening effect, though, due to repulsions between L, the counterions, and the other ligand, and to the softer character of the metal-ligand bond. Thus, in condensed phases where the first coordination shell of the cation is saturated and generally contains neutralizing counterions, the better binding of (alkyl) n PO ligands, compared to (alkyl-O) n PO ones results mostly from "steric" interactions in the first coordination sphere, rather than from the changes in oxygen "basicity". The conclusions are validated by a number of comparisons using polarization functions on the ligand and accounting for correlation effects (MP2 or DFT-B3LYP calculations). They are important in the context of designing efficient ionophores for lanthanide and actinide cations.

The Energetic and Structural Effects of Steric Crowding in Phosphate and Dithiophosphinate Complexes of Lanthanide Cations M3+: A Computational Study

“…As expected from previous studies by our group18–20 the QM calculations show that the hard phosphate ligand generally forms stronger complexes than the softer dithiophosphinate ligand, independent of the hardness of the lanthanide cation. Furthermore, as long as there is still enough space left around the cation, both ligands prefer the smaller harder cations over the larger ones, that is, the order of selectivity is La 3+ <Eu 3+ <Yb 3+ .…”

“…A quasirelativistic ECP of the Stuttgart group was used on the lanthanides, together with the affiliated (5/4/3) valence basis,30, 31 to which one f‐function with an exponent optimized by Frenking et al was added 32. The suitability of this approach, which does not include the effects of electron correlation, for the studied compound types has been discussed in our previous works 16, 18–20…”

Interaction of trivalent lanthanide cations with phosphoryl derivatives, amide, anisole, pyridine and triazine ligands: a quantum mechanics study

“…However, in the presence of counterions, this order is reversed with R PS 3 2 and R PS ligands, the alkyl substituted ligands being 2 2 preferred. Concerning the alkyl vs. O-alkyl substituent effect in R PO complexes, we find that counterions 3 amplify the preference of alkyl substituted ligands [12,13]. Thus, our study shows clearly that counterions have a very strong influence on the ligand-metal interactions, which means that any data regarding the ligand alone, like intrinsic ligand basicity, is insufficient to assess its interactions in condensed phases where the first coordination sphere is saturated.…”

Effect of the nature of substituents at the phosphorus atom on extraction properties of phosphorylketones toward f-elements

“…Steric effect of bulky substituents at the phosphorus atom has an important in fluence on coordination with the metal, which sometimes is even greater than the change in basicity. [18][19][20] It is known that coordination chemistry of lanthanides is greatly influenced by steric hindrance. The changes in the size of secondary (in the terms suggested by A. Platt 20 ) structure of the ligands can considerably change both the composition and the structure of complexes.…”